The information comes from NeoGAF who attended a PS Vita demo event in Osaka, where he picked up a leaflet with the launch information and a FAQ section which explained that the only way to get around this is to reset the console to its factory settings.
Q: How many PSN account can be set up on a Vita system?
A: Only 1 account. If you want to to use a different account, you need to format the system to factory settings.
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